RNA plays an important role in gene expression. It is also known that RNA acts as a catalyst. In addition, RNA interference was found. RNA interference is a phenomenon of mRNA degradation by coupling complementary RNA with a part of the mRNA. Ever since the discovery, there has been a growing need for RNA having a desired base sequence.
RNA has a structure in which ribonucleosides are bound through a phosphodiester group at 5′-position and 3′-position. Therefore, when RNA is chemically synthesized, it is very important that two ribonucleosides are bound through a phosphate group at the 5′-position and 3′-position selectively.
In general, RNA is synthesized by binding a ribonucleotide to a ribonucleoside and then deprotecting the obtained product material. In the above process, the amino group in the base portion, the 2′-hydroxy group and 5′-hydroxy group of the ribonucleotide are protected, and the amino group in the base portion and the 2′-hydroxy group of the ribonucleoside are protected and the ribonucleoside is supported at the 3′-position on a solid-phase support. In the above chemical production process, it is an important point how to efficiently obtain a monomer of which hydroxyl groups are selectively protected. A general synthesis scheme of a monomer for RNA synthesis is described below.

As described above, at least seven steps are needed in order to produce a monomer for RNA synthesis. Specifically, it is relatively easy to selectively protect the amino group in the base portion, since the basicity of the amino group is high. However, in particular, the reactivities of the 2′-hydroxy group and 3′-hydroxy group are similar to one another. Therefore, the number of steps has to be larger, since it is necessary that both of the 3′-hydroxy group and 5′-hydroxy group are simultaneously protected by a cyclic protective group, the 2′-hydroxy group is protected, and then the cyclic protective group at the 3′-position and 5′-position is selectively removed.
Studies of RNA synthesis are focused on the protective group for the 2′-hydroxy group, since the protective group is removed in the final step. For example, Patent Document 1 discloses a ribonucleoside compound of which 2′-hydroxy group is protected by a 1,3-dioxolan-2-yl derivative group and the like. It is described that the substituent can be introduced using an inexpensive reagent and removed under an acidic condition which is inactive against rearrangement of the phosphate diester group. In addition, Patent Document 2 discloses an acetoxymethyl group and the like as a protective group for the 2′-hydroxy group, and it is described that the protective group can be removed by a base while the 3′-5′ bond of RNA is maintained.
Furthermore, non-patent document 1 discloses a method for producing a monomer which is used for RNA synthesis and of which 3′-hydroxy group is H-phosphonate-esterified by carrying out H-phosphonate-esterification without selective control between the 2′-hydroxy group and 3′-hydroxy group and then selectively protecting the 2′-position by TBDMS, i.e. a t-butyldimethylsilyl group.